Method for determining characteristics of an asperity based upon detection members having a unique vibration frequency

ABSTRACT

A sensor head for mapping asperities on a disc surface including a plurality of spaced members having unique excitation characteristics for distinguishing the members excited. The sensor head is designed to fly at pitch angle θ P  above a disc surface via cooperation of an air bearing surface of the sensor head and rotation of the disc to map the asperities on the disc surface. The members are located relative to the air bearing surface for alignment relative to the disc surface. The members fly above the disc surface with the head at various heights. Depending upon the height of an asperity, various members contact the asperity and are excited thereby. Excitation of the various members is detected based upon the unique excitation characteristics for determining the height of an asperity on the disc surface.

REFERENCE TO CO-PENDING APPLICATIONS

This application is a Divisional application of application Ser. No.08/844,836, filed Apr. 22, 1997, which issued Aug. 24, 1999 as U.S. Pat.No. 5,942,680.

Cross-reference is hereby made to U.S. application Ser. No. 08/831,070,filed Apr. 1, 1997, now U.S. Pat. No. 5,825,181 and entitled“Multi-Impact Thermal Asperity of Sensor Head” and U.S. application Ser.No. 08/855,325, filed on May 13, 1997, now U.S. Pat. No. 5,808,184 andentitled “Thermal Asperity Sensor Head with Multiple Spaced AsperitySensors”.

BACKGROUND OF THE INVENTION

The present invention relates to a disc drive storage system. Moreparticularly, the present invention relates to a thermal asperitysensing head design which provides characteristic information forthermal asperity defects on a surface of a magnetic data storage disc.Disc drive data storage devices are known which read and write data froma thin layer of magnetizable material on a surface of one or morerotating discs. Read and write operations are performed through atransducer which is carried in a slider body. Slider and transducer aresometimes collectively referred to as a head. Each disc surface has asingle head associated therewith to read and write data from the discsurface.

Heads are supported via an actuator assembly, which moves the heads foralignment relative to concentric data tracks on a disc surface. Theactuator assembly is controlled by electronic circuitry coupled to theactuator assembly in a known manner. The head is designed to fly abovethe disc surface for operation via cooperation of the rotating discs andan air bearing surface on the slider. As the disc rotates, the discdrags air beneath the air bearing surface of the slider, which developsa lifting force, causing the head to lift and fly above the discsurface.

The entire disc surface of a magnetic disc is not ideal for reading andwriting data. In particular, disc surfaces have asperities whichinterfere with the flying characteristics of the data head, as well asthe read and write operations of the data head. In operation, the headcan come into contact with asperities while the head flies above thesurface of the disc. Potentially, this undesirable contact can causedata written to a particular location on a disc to be lost.

For example, in a magnetoresistive (MR) head which incorporates a MRtype sensor, after contact with an asperity, the heat generated by thecontact changes the resistive properties of the MR sensor. As a result,the corresponding signal read by the MR head is distorted by a voltagespike and subsequent decay, sometimes causing the data stored near theasperity to be unrecoverable. The voltage spike in the read signal isfrequently referred to as a “thermal asperity”, while the defect on thedisc is referred to as an “asperity”. However, since one is indicativeof the other, the two terms are frequently used interchangeably.

Disc asperities which are located in the factory during a defectscanning process can be recorded in a disc drive's primary defect listso that the drive does not store data at those locations. Known asperitydetection techniques use sensors (such as MR sensors or piezoelectricsensors). Such known asperity detection techniques rely both on theflying characteristics of the heads and upon the thermal response fromfriction induced head/asperity contact. The energy of the impact oramplitude detected by an MR or other sensor is calibrated to determinethe asperity characteristics such as height of the asperity. Bycalibrating the slope and duration of the resistance change waveform toa range of asperity heights and characteristics, the height of aparticular asperity can be determined by detecting the momentary changein resistance of the sensor after contact.

However, the voltage signals corresponding to the impact of a sensorelement with an asperity include components of noise, air bearingexcitation, and other vibrations or excitations which may detract fromthe accuracy of calibrating the height of an asperity based upon thevoltage signal from an MR sensor element or a piezoelectric sensorelement after contact with the asperity.

Additionally, such devices require that the disc surface be scanned atvarious fly heights of the head so that various sizes of asperities canbe detected to map the entire range of defects. As the speed of rotationof the disc is changed, the response of the specially-designed headsalso changes. For example, if the speed is reduced, the energy of impactis reduced, thus making it more difficult to calibrate the defect sizeand height.

SUMMARY OF THE INVENTION

The present invention relates to a sensor head for detecting anddetermining the characteristics of asperities on a disc surface. Thesensor head includes a body having a leading edge, a trailing edge andan air bearing surface. A plurality of spaced members are located on theair bearing surface. Each of the members has a unique excitationcharacteristic. The members are arranged at various heights above thedisc surface when the head is flying above the disc surface at a pitchangle θ_(P).

Asperities may extend above the disc surface at different heights.Certain members will contact the asperity depending upon whether the flyheight of the member above the disc surface is lower than the height ofthe asperity. The unique excitation characteristics of the members areused to distinguish the members which contact the asperity. The flyheight above the disc surface of the members contacted is used tocalculate the height of the asperity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a sensor head of the present invention.

FIG. 2 is diagrammatic view of the sensor head and disc surfaceillustrating operation of the sensor head to detect asperities on thedisc surface.

FIG. 3 is a diagrammatic view illustrating vibration of frequencymembers in response to contact with an asperity.

FIG. 4 is a diagrammatic view illustrating calculation of asperityheight and characteristics based upon contact with frequency members.

FIG. 5 is a detailed perspective view of the construction of frequencymembers on the sensor head.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a sensor and burnish head 10 shown inFIG. 1 which is supported to fly above a disc surface in a known mannerfor mapping disc defects. The sensor head 10 shown in FIG. 1 includes alower air bearing surface 12, a leading edge 14, a trailing edge 16, andan upper surface 18. The sensor head 10 is supported relative to a discsurface via a suspension system (not shown) coupled to the upper surface18. The sensor head 10 is supported via the upper surface 18 so that thelower air bearing surface 12 faces the disc surface to provide an airbearing for lifting the sensor head 10 to fly above the disc surface incooperation with the rotation of the disc.

As shown in FIG. 1, the air bearing surface 12 includes the side rails20 a-b, a center rail 22 and sloped leading surface 24. Recessedpressure cavities 25 a-b are defined between the side rails 20 a-b andcenter rail 22. The side rails 20 a-b and center rail 22 definehigh-pressure surfaces. Rotation of a disc provides a flow of air fromthe sloped leading surface 24 along the air bearing surface 12 forcreating a hydrodynamic lifting force to lift the sensor head 10 to flyacross the disc surface.

Center rail 22 includes a plurality of spaced frequency members 26 a-d,positioned at a trailing end 16 of the center rail 22 for alignment withthe disc surface as the sensor head 10 flies above the disc surface.Each frequency member 26 a-d is designed to resonate at a unique anddifferent frequency when excited. The frequency members 26 a-d aredesigned for detecting and calculating the characteristics (height) ofan asperity while the sensor head 10 is flying above a disc surface aswill be described.

FIG. 2 is a side elevational view of the sensor head 10, greatlyexaggerated, illustrating operation of frequency members 26 a-d forascertaining the characteristics of asperities on a disc surface 28. Asshown, disc surface 28 includes asperities 30 a and 30 b. Asperity 30 bis higher than asperity 30 a. Sensor head 10 flies above the discsurface 28 at a pitch angle θ_(P) at a fly height of H_(fly). If theheight of the asperities 30 a and 30 b is greater than H_(fly) of thesensor head 10, sensor head 10 will contact the asperities 30 a and 30b.

Preferably, frequency members 26 a-d are polygonal-shaped members havingan end integrally formed with the slider 12 and a cantilevered end 31 todefine a spring-like element. Frequency members 26 a-d extend along alength of the air bearing surface 12 of the sensor head 10 separated byrecessed cavities 32 to a portion of the air bearing surface 12.Preferably, form frequency member 26 a is located adjacent to thetrailing end 16 and members 26 b-d extend along the air bearing surface12 therefrom. During operation, the elevation of each of the frequencymembers 26 a-d is different since the sensor head 10 flies at a pitchangle θ_(P). Thus, frequency member 26 a is flying closer to discsurface 28 than frequency member 26 d based upon the pitch angle θ_(P).Thus depending upon the height of the asperity, the asperity willcontact one or more of the frequency members 26 a-d based upon θ_(P) ofthe sensor head 10.

When the frequency members 26 a-d contact an asperities, the impact ofthe contact will cause the frequency members 26 a-d to vibrate. Inparticular, contact with an asperity will excite the cantilevered endsof the frequency members 26 a-d causing the frequency members tovibrate. Each of the frequency members 26 a-26 d has unique dimensionsto define a unique vibration frequency based upon the spring constant ofthe frequency member 26 a-d. In particular, frequency member 26 avibrates at F_(a), frequency member 26 b vibrates at F_(b); frequencymember 26 c vibrates at F_(c) and frequency member 26 d vibrates atF_(d).

Piezoelectric transducer elements and associated contacts 34 a-34 d arecoupled to each of the frequency members 26 a-26 d to measure thevibration of frequency members 26 a-26 d and produce a signalcorresponding to the vibration of the frequency members 26 a-26 d.Piezoelectric elements 34 a-34 d are coupled to a PZT signal monitoringdevice 36. The piezoelectric signals produced from piezoelectricelements 34 a-34 d are analyzed by the PZT signal monitoring device 36in a known manner to determine the frequency of the signal generated bypiezoelectric elements 34 a-34 d in response to vibration of frequencymembers 26 a-d. Since each frequency members 26 a-26 d vibrate at aunique frequency, the signals analyzed are used to isolate theparticular frequency members 26 a-d excited by contact with an asperity30 a and 30 b.

In a preferred embodiment of the invention, the highest flying frequencymember 26 d is designed to have the lowest F_(d) and the frequencymembers 26 c-a below frequency member 26 d have successively higherfrequencies F_(c-a) so that the lowest detected frequency corresponds tothe highest frequency member contacted and thus corresponds to theheight of the asperity. A height calculation processor 38 is coupled tothe PZT signal monitoring device 36 to calculate the height range of anasperity based upon the frequency members excited. Although apiezoelectric detection element is described, the invention is notlimited to a particular vibration detection device, and alternateembodiments of the invention may use other vibration detection devices.

FIG. 3 illustrates vibration for frequency members 26 a-26 c via contactwith an asperity. As shown, frequency member 26 a contacts an asperityfirst, followed by frequency member 26 b and then frequency member 26 c.As shown, each frequency member 26 a-c produces a unique vibrationfrequency F_(a)-F_(c) so that vibration of the various frequency members26 a-c may be distinguished. The height calculation processor 38estimates the height of the asperity based upon the highest frequencymember 26 a-c vibrated. As shown in FIG. 4, the height of an asperity isbased upon H_(fly) plus the height of the highest frequency membercontacted, i.e. h_(a), or h_(b) or h_(c) or h_(d). The followingequations are used to estimate the Height of an asperity based upon thehighest frequency member 26 a-d contacted.

For frequency member 26 a:

H _(fly)<Height<H _(fly)+(L _(a) +S)×SIN(θ_(P))  Equation 1

For frequency member 26 b:

H _(fly)+(L _(a) +S)×SIN(θP)<Height<H _(fly)+((L _(a) +S)+(L _(b)+S))×SIN(θ_(P))  Equation 2

For frequency member 26 c:

H _(fly)+((L _(a) +S)+(L _(b) +S))×SIN(θ_(P))<Height<H _(fly)+((L _(a)+S)+(L _(b) +S)+(L _(c) +S))×SIN(θ_(P))  Equation 3

For frequency member 26 d:

H _(fly)+((L _(a) +S)+(L _(b) +S)+(L _(c) +S))×SIN(θ_(P))<Height<H_(fly)+((L _(a) +S)+(L _(b) +S)+(L _(c) +S)+(L _(d)+S))×SIN(θ_(P))  Equation 4

Where,

L_(a), L_(b), L_(c), and L_(d) relate to the length L of the frequencymembers; and

S relates to the length of the recessed cavities 32 between frequencymembers 26 a-d.

The pitch angle θ_(P) of sensor head 10 is determined based upon thespeed of rotation of the disk, the characteristics of the air bearingsurface 12, and a load force applied to an upper surface of the sensorhead 10 via a load beam (not shown) in a known manner.

Thus, as shown in FIG. 2, frequency member 26 a will contact asperity 30a, but none of the other frequency members 26 b-d will contact asperity30 a. Thus, the height of asperity 30 a is calculated based uponEquation 1. Asperity 30 b extends to a higher elevation above the discsurface 28, than asperity 30 a. Frequency members 26 a-26 c contactasperity 30 b and thus, the height of asperity 30 b is calculated basedupon Equation 3.

The asperity height information may be downloaded to a data storagedevice, such as a RAM memory device in the control circuitry (not shown)in a known manner. The asperity information is stored, relative to disclocation of the asperity to produce an asperity map. It should be notedthat while only four frequency members 26 a-d are shown, any number offrequency members 26 may be used for determining asperity height.Although a cantilevered type spring element has been described, theinvention is not so limited, and alternative elements which produce aunique distinguishable signal when contacted may be employed toimplement the present invention.

The sensor head 10 of the present invention also provides a burnishingsurface for removing the asperity. Due to the abrupt features of thefrequency members 26 a-d and recessed cavities 32, the sensor head 10 ofthe present invention provides high contact stress between the frequencymembers and the asperity to burnish the asperity.

FIG. 5 is a detailed perspective view of cantilevered polygon-shapedfrequency members 26 a-c (only frequency members 25 a-c are shown)defined by dimensions L, W, and H. The dimension of individual frequencymembers 26 a-d is varied to define unique vibration frequencies F_(a),F_(b), F_(c), and F_(d) (i.e. spring constant) for members 26 a-26 d. Inthe embodiment of the sensor head 10 illustrated in FIG. 5, dimensions Land W are varied and dimension H remains constant to define uniquevibration frequencies for individual frequency members 26 a-d.Preferably, dimension H is the same for all frequency members 26 a-d sothat the cantilevered ends 31 lie in the same plane to facilitatecalculation of the height of the members 26 a-d. Either dimension L orW, alone or in combination, may be varied to define the unique vibrationfrequencies for polygon members 26 a-d.

The table below illustrates various excitation frequencies for frequencymembers 26 a-26 d, for a series of frequency members having constant Wand H dimensions but a varied L dimension.

Example

Minimum Frequency Width Height Length Frequency Member (μm) (μm) (μm)(kHz) 26a 200 20 9 189 26b 200 20 7 146 26c 200 20 5 105 26d 200 20 3 63

Thus, the excitation frequencies detected by the PZT signal monitoringdevice 36 are compared with the known frequencies listed in the tableabove for frequency members 26 a-26 d to determine the frequency members26 a-26 d excited for calculating the height of an asperity aspreviously explained. The distance between frequency members 26 a-d isclosely controlled in order to achieve a desired asperity heightdetection sensitivity or resolution for pitch angle θ_(P).

The number of spaced frequency members, and extent of the air bearingsurface along which the frequency members extend, is defined to providea sufficient range of members to detect asperities at various heights.The height of the asperity can be determined by the first frequencymember contacted and thus asperity detection and analysis is noteffected by shifts in flying parameters of the slider after contact withan asperity.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. A method for determining characteristics of anasperity on a surface of a disc comprising the steps of: providing ahead including a body having a leading edge, a trailing edge, and an airbearing surface, said air bearing surface including a plurality ofspaced detection members, each of the plurality of detection membershaving a unique vibration frequency upon excitation to distinguish eachof the plurality of detection members excited by contact with an object,each of the plurality of detection members being coupled to a detectiondevice to detect vibration of each of the plurality of detectionmembers; flying the head above the surface of the disc at a pitch angleθ_(P) and a height H_(fly) as the disc is rotated; detecting excitationof each of the plurality of detection members excited by contact with anasperity based upon the unique vibration frequency of each of theplurality of detection members; calculating a height of the asperitybased upon a height of excited detection members above the disc surface.2. The method of claim 1 wherein the plurality of detection members arecantilevered members which are separated by recessed cavities and theplurality of detection members extend from the trailing edge of the headalong the air bearing surface and are arranged from a highest vibrationfrequency to a lowest vibration frequency and the step of detectingexcitation detects the lowest vibration frequency and calculates theheight of the asperity based upon the height above the disc surface ofthe detection member corresponding to the lowest vibration frequencydetected.
 3. The method of claim 1 wherein the height of the asperity iscalculated based upon H_(fly)±(L+S)×SIN(θ_(P)) for each of the pluralityof detection members excited.